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One- shot 3D infusion moulding of hulls

News International-French

The SCRIMPTM System was first introduced to Taiwan in early 1999 through the United Ship Design & Development Centre (USDDC). Horizon Yachts subsidiary ATech Composites is the first SCRIMPTM licensee in Taiwan. ATech has been applying infusion technology to all sizes of hulls, decks and superstructures for the past five years. Today, the infused resin flow is easily controlled in three dimensions to wet through hulls as long as 130 feet. Single-skin or sandwich shell plates andthe main longitudinal and transverse structures are infused in one shot. Here, we present several successful cases for lengths from 48 feet to 130 feet.

With the traditional infusion process, resin flow in X-Y (2D) directions can be easily controlled whether resin channels or distribution media are used with the core material (fig.1). The main manufacturing problems actually begin during the grinding stage that follows the infusion process. Grinding is necessary for secondary bonding, especially with unsaturated polyester (UP) resins, but it is a dirty, laborious process. Stress flow discontinuity can occur with secondary bonding, due to non-uniform mechanical properties between plates and structures (fig.2).

To address this structural problem, the best solution is to infuse the whole structure in one piece, or at least including the main g i rders and web frames. This reduces the grinding surface to a limited area at BHD or deck joiner edges. One-shot infusing additional parts will save more weight and yield improved, more uniform quality. The resulting structure also offers adequate structural strength and meets classification requirements, even with inexpensive UP resin. One-shot infusion demands more precise control of the resin flow, so a more sophisticated system like the SCRIMPTM process must be used. An ord i n a ry core-distributing system does not control resin flow sufficiently.

More than a hundred complicated-surface parts such as cockpit bulwarks and stairways have been produced successfully over the past 5 years, encouraging ATech to move on to one-shot infusion with stiffeners and pre-inserted fittings.

Infusion flow control for three-dimensional structures

The first challenge with 3D infusion is the uneven length of resin path when passing through hat stiffeners (figs.4 and 5). When resin flows to the web edge of a girder, it splits into two paths, climbing up through the girder web and penetrating underneath the stiffener. Both paths must be controlled if they are to meet up on the other side of the girder web edge within the same time frame, to allow the resin front to flow to the next girder without leaving resin-starved areas behind.

The key is to delay the flow rate at the bottom plate and speed up the resin front on the stiffener surface. The first one can be achieved by inserting a narrow piece of distribution media underneath the girder to create a “breaking area” on both sides (fig.6). For the top path area, two or three more resin feed pipes will accelerate resin flow.

SCRIMP up to 130 feet in 3D

After successfully using the SCRIMPTM process for a 127-ft boat (hull no.125-010, fig.8), we decided to try to produce our 115-ft boat (hull no.112-011, fig.10) in full 3D infusion.

The 127-ft hull was produced using hand la mination of the sideshell outer skin and all main girders. The side sandwich panel and bottom single-skin plate were produced by infusion. 0The 1 1 5 - f t hull was produced entirely using SCRIMPTM, including the main girders. Both designs are DNV approved, with the same scantling and lamination schedule, materials and workers.

Table 1 shows that for the 115-ft hull, using the SCRIMPTM process for the shell plate and the main girders in one shot can save 16% in overall product weight and about 5% in labour cost. Infusion time depends mainly on the beam (or hull girder length). Producing a 48-ft hull by SCRIMPTM takes three hours, while a 130-footer only requires five hours.

Table 1: weight and labour cost comparison for two 12-ft yacht hulls produced using different processes.

Hull number

S112011

S125010

LOA (ft)

115

127

L (m)

34.96

38.7

B (m)

7.95

8.03

Girder length at midship (m)

15.04

15.57

Actual hull weight (kg)

29.975

42.061

Man hours (lamination)

7.091

8.338

Wt. scale to same GxL

35.187

42.061

Weight ratio

84%

100%

Man hours / LxB

25.51

26.83

Labour ratio

95%

100%

Benefits of one-shot 3D infusion for shell and girders

We all know the general benefits of the infusion process: weight saving, improved strength, less pollution, better working environment, etc. But there is always more work than with hand lay-up, such as laying infusion materials, tearing materials off, grinding, etc.

Focus......

The general benefits of the infusion process: weight saving, improved strength, less pollution, better working environment, etc.

The infusion pro ce ss is a growing trend for marine co m p o s i tes. The co st is lower for larger structures, the mechanical properties are uniform, and the products are lighter.

Traditional hand lay-up actually offers higher efficiency and lower cost than high-tech infusion, especially for smaller size vessels. We compared hulls from 48 feet to 120 feet (table 2) in terms of working efficiency (dry-fibre and core-material weight laid per man hour) for the following three combinations: 1) hull plate and main girders produced by hand lay-up; 2) hull plate p roduced by SCRIMPTM (2D) and girders by hand lay-up; 3) shell plate and girders produced by SCRIMPTM (3D).

Table 2.: lamination efficiency of different processes from 48 to 120 feet.

Yacht hull size, length in feet

Process

Hull components

48

64

76

82

120

1) Hand lay-up

Shell plate

1.78

1.74

1.65

1.75

1.78

Main girders

3.79

3.7

3.8

3.27

3.03

Shell + main girders

2.38

2.23

2.08

2.05

2.03

2) SCRIMP (2D) + hand lay-up for main girders

Shell plate

2.46

3.14

4.36

4.07

3.76

Main girders

3.79

3.7

3.8

3.27

3.03

Shell + main girders

2.86

3.28

4.25

3.91

3.76

3) SCRIMP(3D)

Shell plate (2D)

2.46

3.14

4.36

4.07

3.70

Shell + main girders (3D)

2.14

2.93

3.36

3.99

3.70

Cost comparison 3) vs. 2)

Labour cost

25.1%

10.7%

20.9%

-2.0%

-2.4%

Material cost

3.0%

2.5%

2.0%

2.0%

1.5%

Total direct cost

10.5%

5.1%

8.4%

0.4%

0.0%

Cost comparison 3) vs. 1)

Labour cost

10.2%

-31.4%

-61.5%

-94.3%

-82.3%

Material cost

11.2%

10.5%

10.0%

9.5%

9.0%

Total direct cost

9.7%

-5.2%

-16.0%

-27.8%

-23.8%

Hand lay-up vs. SCRIMPTM (3D)

For a number of reasons, the first solution using hand lay-up cannot compete with the SCRIMPTM ( 3D) process when it comes to a hull length over 55 feet. It is not easy to use wet lay-up on the vertical hull sides, while whole rolls of fibre can be easily laid using dry lay-up – although when boat size is over 90 feet, the deeper hull side makes that a bit more difficult.

To get good wet out in hand lay-up, the fibre weight is limited, while heavier fibres (up to 3,000g/m2) can be used in dry lay-up. Even though the extra materials for SCRIMPTM cost 9% to 11% more, depending on hull length, the labour cost savings are higher, so the overall cost is kept low for solution 3.

SCRIMPTM (2D) + hand lay-up vs. SCRIMPTM (3D)

Solution 2 using SCRIMPTM (2D) + hand lay-up is more costeffective than 3D infusion, mainly because setting up indirect materials for SCRIMPTM becomes much more complicated. But for hull lengths 82 feet and more, there is not much difference in cost. Table 2 compares direct labour and material cost only, but there are still more invisible benefits for 3D infusion, such as consistent product quality for the hull and girders; less harm to the environment than with hand lay-up; economic use of resin, creating lighter and faster boats; and shorter manufacturing t i m e s , because the dry lay-up of the hull’s shell and girders can be done separately at the same time in diff e rent shops. This was especially the case for production boats. The average de-moulding time of a standard 82-ft hull is 20 days, while the traditional 2D plate infusion method requires 38 days, almost twice as long.

Conclusion

The infusion process is a growing trend for marine composites. As this paper shows, the main girders and hull can be one-shot moulded using SCRIMPTM with distribution media and skill to control the resin front. The cost is lower for larger structures, the mechanical pro p e rties are uniform, and the products are lighter. Even designs with single-skin bottoms to save money on expensive core materials and with cheap UP resin can still meet classification requirements. After all, in the general motoryacht industry, we want our process to be profitable (fig.10), not just high-tech!